Although the design life of a wind turbine gearbox is typically more than twenty years, failures of wind turbine gearboxes within four to five years are not uncommon. This is because residual useful life (RUL) calculation procedures are based on assumed operating profiles, whereas in operation, the actual profile could be very different.
Monitoring operating parameters related to the operation of a wind or water turbine or component thereof, and determining when these parameters move outside an operating window, may indicate that some kind of maintenance or investigation is needed. Operating parameters that are monitored could include lubrication temperature, lubrication debris, vibration, and power output.
Vibration is commonly measured by Condition Monitoring Systems. Generally speaking, large vibrations compared to a norm is indicative of damage.
Vibration analysis generally relies on a measurement provided by a sensor exceeding a predetermined threshold, which is prone to false alarms if the threshold is set too low. The threshold level is not necessarily constant and may vary with frequency (and hence speed). The presence of shocks and extraneous vibrations means that the threshold level must be set sufficiently high to minimise the risk of false-alarms. Furthermore, the threshold must be sufficiently high to avoid any negative effects caused by ‘creep’ in sensor performance which may occur over its lifetime. In addition, there is no discrimination between vibrations associated with failure or damage and those which are not indicative of failure or damage.
Faults developing during operation, such as an imbalance in a rotor, can create loads on a bearing in excess of that expected resulting in a reduction in its design life. Incipient faults, such as unbalance, can be detected from analysis of vibration signatures. This gives the magnitude of an imbalance, and an excitation force due to imbalance is a function of the magnitude of the imbalance and square of the speed. An excitation force due to faults can thus be calculated from field operational conditions and used to calculate individual component loads. Deviation from the assumed operating profile can be addressed by using a generic wind simulation model to determine load at the turbine shaft, which allows individual component loads based on the field operational conditions to be calculated. Combining these gives the total load at each component, which can be is used to estimate the remaining life of the individual components and the life of the gearbox.
However, shortcomings in wind simulation models mean that the load at the turbine shaft may not be reliably or accurately determined.
Equivalent Operating Hours (EOH), in simple terms, defines damage as being equivalent to the damage caused to a wind or water turbine or components thereof by one hour of operation under rated operating conditions. The EOH is equal to a weighting factor related to the operational condition multiplied by a duration (or alternatively, frequency) of that condition. For any operation in which damage caused is the same as that expected to be caused under rated conditions, the EOH of a component after 1 h will be 1 h, and the weighting factor will be 1.0. If an operational event causes greater damage, then the EOH will be reduced accordingly. Thus, an operational event of duration of 0.2 h of duration and having a weighting factor of 0.7, then the EOH after 1 h will be 0.8×1+0.2×0.7=0.94.